I brew my own beer. I'd like to use Arduino to help me.For the most part, I just want to detect temperature and specific gravity (density) of my beer (wort).

I've already got the temperature side worked out (simple thermistor and a voltage divider on an analog line), but I'm having trouble tracking down a suitable sensor for liquid density (specific gravity).

Any suggestions on where to get a sensor (preferably a handful of them) for measuring this?

Well, that sort of works. The thing with fermenting beer is that sediment falls out of solution during fermentation, which wouldn't show up in the density measurement, but would show up by measuring the mass of the fermentation vessel.

CO2 gas does escape though an airlock (a one way valve), but I think I'd need some way to account for sediment (not a homogeneous solution).

www.micromotion.com has a density sensor, but they quoted me $5000-6000 for the sensor. I'm looking to pay $50.

Well can't you just take out a fixed volume, say 10mL, and weight that?

If you want fancy continuous monitoring you need two pressure ports in the fluid, one at a certain height above the other one, and connect the two ports to a differential pressure sensor. Since beer is incompressible, you can then use the hydrostatic equation:

you could try using a capacitance sensor to measure the density of the wort. i've used a qprox qt300 capacitance-to-digital sensor with arduino for a completely different purpose, but humidity and density sensing is apparently what they are designed for - they're used quite widely in manufacturing industry... and it's pretty easy to get the qt300 to talk to arduino. it returns arbitrary 16-bit readings, so you will have to calibrate it initially by using the usual hydrometer (sp?) method, but provided you feed it a nice smooth power supply (pref batteries) the readings stay pretty rock solid.

actually, even if you don't calibrate your SG readings against the sensor, you could just get arduino to log the sensor readings every few hours or so and let you know when they become stable at the end of fermentation. if you really want to know your alcohol content you could just do the initial and final SG readings manually.

i live in melbourne, australia, and ordered a few qprox chips from farnell in sydney (only cost about A$9 or US$7 each i think). outside oz i think they are widely available - spark fun should have them.

Sorry for the confusion. You could probably measure density with a capacitance sensor (higher gravity wort is more or less capacitive than beer (or lower gravity wort).. or alcohol). Not sure how accurate this will be.

Might be worth a try.

Anyway, I only mention that other capacitance sensor because that's also useful for my mini brewery.

Hmm... I'm curious, how does the capacitance sensor measure density? Would it work in the air?

a) they measure the total capacitance of all material in proximity to the sensor electrode. this material's capacitance is determined by its dielectric constant multiplied by how much of it there is. since wort is heavier at the start of fermentation than it is at the end, i'd guess that its dielectric constant changes too. that's why i suggested a capacitnce sensor.

b) yep, they will work through air quite well, although quantum for some reason stress that that isn't their designed function. i've used them to build a theremin, to quite good effect. it's worth bearing in mind that the design of the sensor electrode ("antenna") has a large bearing on the performance of the system. in the case of my theremin, i had to use a system of antennas (actually a slightly modded TV rabbit-ear setup) to increase the sensing distance to beyond about 300mm. obviously, this won't be an issue for the wort density sensor.

s: you might need to be careful about bubbles sticking to the electrode...? also, keep in mind that the the qt300 isn't good for single-shot measurements - the first few values returned are always inaccurate. the trick is to request about 100 readings in rapid succession, and use the average (or sum) of the last 20 or so as your data.